Power meters used to detect the power consumption of electric appliances and facilities at homes and in industry are categorized into induction-type power meters and electronic power meters. Although induction-type power meters comprising rotating disks were conventionally predominant, the electronic power meters are recently finding wider use due to the development of electronics. Power meters adapted to conventional standards such as IEC62053-22, etc. cannot conduct the accurate detection of distorted-waveform current such as half-wave, sinusoidal, alternating current, etc., failing to measure power accurately. Accordingly, IEC62053-21, a power meter standard adapted to distorted waveforms (half-wave rectified waveforms), was enacted in Europe. In other countries than those in Europe, too, power meters such as present rotating-disk meters, etc. failing to accurately measure the power of distorted waveforms were discarded, and power meters adapted to IEC62053-21, which use current transformers (CTs) or Hall elements for current detection, have been being put to actual use. In the industrial fields such as inverters, etc., too, the current transformers have important role in the detection of distorted-waveform alternating current and direct-current-superimposed alternating current.
A current sensor using a Hall element comprises a Hall element disposed in a gap of a magnetic core, and a conductive wire for flowing current to be measured, which penetrates through a closed magnetic circuit of the magnetic core, to detect a magnetic field generated in the gap, which is substantially proportional to the current, by the Hall element, thereby detecting the current.
The current transformer (CT) usually comprises a magnetic core having a closed magnetic circuit, a primary winding for flowing current to be measured, which penetrates through the closed magnetic circuit, and a secondary winding in a relatively large number of turns. FIG. 8 shows the structure of a current transformer (CT)-type current sensor. The magnetic core is in a ring-type or assembled core-type shape, and a ring-type, toroidal core with windings can be made smaller with a reduced magnetic flux leakage, thereby enabling performance near a theoretical operation.
Ideal output current i obtained from alternating through-current I0 under the condition of RL<<2πf·L2 is I0/N, wherein N is the number of a secondary winding, and output voltage E0 is I0·RL/N, wherein R is load resistance. The output voltage E0 is actually smaller than the ideal value due to a core loss, a leaked magnetic flux, etc. The sensitivity of the current transformer corresponds to E0/I0, but this value is actually determined by a coupling coefficient of primary and secondary windings. E0=I0·RL·K/N is satisfied, wherein K is a coupling coefficient.
Although the coupling coefficient K is 1 in an ideal current transformer, K is about 0.95-0.99 in actual current transformers at RL of 100Ω or less, under the influence of the internal resistance of windings, load resistance, a leaked magnetic flux, the non-linearity of a permeability, etc. Because the K value is low if there is a gap in a magnetic circuit, a toroidal core with no gap can provide an ideal current transformer having the largest degree of coupling. The larger cross section area S, the larger number N of a secondary winding, and the smaller load resistance RL provide the K value closer to 1. This K value also varies depending on the through-current I0. In the case of micro-current I0 of 100 mA or less, the K value tends to be low. Particularly when a low-permeability material is used for the magnetic core, this tendency is large. Accordingly, when the micro-current should be measured at high accuracy, a high-permeability material is used for the magnetic core.
A ratio error is a relative error of the measured value to the ideal value at each measurement point, indicating how the measured current is accurate. The coupling coefficient is correlated with the ratio error. A phase difference represents the accuracy of a waveform, indicating the phase deviation of the output waveform from the original waveform. The current transformer output usually has a leading phase. These two characteristics are particularly important to the current transformers used for integrating power meters, etc.
In the current transformer that should measure micro-current, materials having high initial permeability such as Parmalloy, etc. are generally used to have a high coupling coefficient K, and small ratio error and phase difference. The maximum through-current I0max of the current transformer is defined as the maximum current with secured linearity, which is affected by load resistance, internal resistance, and the magnetic properties of core materials used. To enable the measurement of large current, the core materials preferably have as high saturation magnetic flux density as possible.
Known materials used for the current transformer cores include silicon steel, Parmalloy, amorphous alloys, Fe-based, nano-crystalline alloys, etc. Because inexpensive, high-magnetic-flux-density silicon steel sheets have low permeability, large hysteresis, and poor magnetization loop linearity, they suffer largely varying ratio error and phase difference, resulting in difficulty in providing high-accuracy current transformers. Further, having a large residual magnetic flux density, they cannot easily conduct the accurate measurement of unsymmetrical current such as half-wave, sinusoidal, current, etc.
The Fe-based amorphous alloys suffer large variations of a ratio error and a phase difference when used for the current transformer. JP 2002-525863 A discloses that because a Co-based, amorphous alloy heat-treated in a magnetic field has good magnetization curve linearity and small hysteresis, it exhibits excellent characteristics when used for a current transformer (CT) for detecting unsymmetrical-waveform current. Co-based, amorphous alloys having as low permeability as about 1500 and good magnetization curve linearity are used for current transformers (CTs) for current detection, which are adapted to the above IEC62053-21, a standard of power meters. However, the saturation magnetic flux densities of the Co-based, amorphous alloys are insufficiently as low as 1.2 T or less, and they are thermally unstable. Thus, there are problems as follows: the measurement is limited when biased with large current; they are not necessarily sufficient in size reduction and stability; and because their permeability cannot be increased so high from the aspect of magnetic saturation in view of direct current superposition, they have large ratio error and phase difference, important characteristics for current transformers. In addition, the Co-based, amorphous alloys are disadvantageous in cost because they contain a large amount of expensive Co.
Materials having relatively high permeability such as Parmalloy, etc. are used for current transformer cores in integrating power meters adapted to the conventional standards of IEC62053-22, etc. Such high-permeability materials can measure the power of positive-negative-symmetrical current and voltage waveform, but cannot measure the power of unsymmetrical-waveform current and distorted-waveform current accurately.
The Fe-based, nano-crystalline alloys having high permeability and excellent soft magnetic properties are used for magnetic cores of common-mode choke coils, high-frequency transformers, pulse transformers, etc. The typical compositions of the Fe-based, nano-crystalline alloys are Fe—Cu—(Nb, Ti, Zr, Hf, Mo, W, Ta)—Si—B, Fe—Cu—(Nb, Ti, Zr, Hf, Mo, W, Ta)—B, etc. described in JP 4-4393 B and JP 1-242755 A. These Fe-based, nano-crystalline alloys are usually produced by forming amorphous alloys from a liquid or gas phase by rapid quenching, and heat-treating them for micro-crystallization. It is known that the Fe-based, nano-crystalline alloys have as high saturation magnetic flux density and as low magnetostriction as those of the Fe-based amorphous alloys, meaning excellent soft magnetic properties. JP 1-235213 A, JP 5-203679 A and JP 2002-530854 A describe that the Fe-based, nano-crystalline materials are suitable for current sensors (current transformers) used in leakage circuit breakers, integrating power meters, etc.
However, current transformer cores made of high-permeability materials such as conventional Parmalloy and Fe-based, nano-crystalline, soft-magnetic alloys full to detect current sufficiently because of magnetic saturation, particularly in the case of direct current bias. The cores of the Fe-based, nano-crystalline, soft-magnetic alloys having high saturation magnetic flux density and permeability are suitable for current transformers such as leakage circuit breakers, etc., but they have so small HK that they cannot easily measure current in the case of direct current bias because of their magnetic saturation. In the case of a current transformer used for half-wave, sinusoidal, current, direct current of Imax/2 π is superimposed, where Imax is a peak value of the half-wave, sinusoidal, current. Accordingly, the current transformer cores made of the conventional Fe-based, nano-crystalline, soft-magnetic alloys described in JP2002-530854 A, etc., which have as high permeability as 12000 or more, are magnetically saturated because of direct-current magnetic field bias. Thus, they are not suitable for the measurement of such unsymmetrical-waveform current.
Demand has thus become mounting for a magnetic material making it possible to measure the power of unsymmetrical-waveform current accurately. Even when unsymmetrical-waveform current such as half-wave, sinusoidal, current and direct current are superimposed, the accurate measurement of alternating current is demanded. Necessary to meet such demand is a current transformer core made of a magnetic material having a low residual magnetic flux density, small hysteresis, and good magnetization curve linearity, which is not easily saturable and generates a relatively large anisotropic magnetic field HK.